1. Field of the Invention
The present invention relates to positive active materials composed of a compound oxide of lithium and a transition metal. This compound oxide has a layered structure. The present invention also relates to nonaqueous electrolyte secondary batteries produced using the positive active materials. In particular, the present invention relates to a positive active material having excellent high-temperature characteristics and a nonaqueous electrolyte secondary battery produced using the positive active material.
2. Description of the Related Art
Various portable electronic devices such as camcorders, cell phones, and laptop computers have now been commercially available. Rapidly increasing demand for such devices involves further reductions in the size and weight of these devices. Such reductions require batteries, particularly secondary batteries, having a higher energy density for use as portable power sources. Currently, such batteries have been intensively researched and developed. Among them, lithium-ion secondary batteries, which involve doping and undoping with lithium ions, have a higher energy density than known aqueous electrolyte secondary batteries such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries. Lithium-ion secondary batteries are now in high demand; they are expected to expand their range of application with improving environmental resistance.
Examples of positive active materials that have been put to practical use in lithium-ion batteries include lithium-cobalt oxide and lithium-nickel oxide, which have a layered rock-salt structure, and lithium-manganese oxide, which has a spinel structure. These positive active materials have both advantages and disadvantages. At present, lithium-cobalt oxide is extensively used for its best-balanced characteristics such as capacity, cost, and thermal stability. On the other hand, lithium-manganese oxide has low capacity and slightly poor storage stability at high temperature. Also, lithium-nickel oxide has slightly low stability in crystal structure, poor cycle characteristics, and poor environmental characteristics, particularly high-temperature characteristics. However, lithium-manganese oxide and lithium nickel oxide, each providing less material cost and more stable supply than lithium-cobalt oxide, have been expected and researched.
Examples of known proposals involving lithium-nickel oxide include the partial replacement of nickel contained in the oxide with another element to improve the cycle characteristics (for example, see Japanese Unexamined Patent Application Publication Nos. H08-37007 and 2001-35492), the addition of a certain material such as a metal salt (for example, see Japanese Unexamined Patent Application Publication No. H07-192721), and the use of a certain binder in a positive mixture (for example, see Japanese Unexamined Patent Application Publication No. H10-302768). Investigation by the present inventors, however, shows that the above proposals cannot sufficiently improve the environmental characteristics, particularly the high-temperature characteristics.
Another proposal is the coating of a positive active material composed of lithium-nickel oxide with a single material such as a conductive material and another oxide having a layered structure (for example, see Japanese Unexamined Patent Application Publication Nos. H07-235292, H11-67209, H11-283623, and 2000-149950). Investigation by the present inventors shows that this proposal cannot improve the high-temperature characteristics of the positive active material.
A further proposal is the coating of a compound oxide of lithium and a transition metal with another compound (for example, see Japanese Unexamined Patent Application Publication Nos. H08-222219, 2000-149948, and 2002-164053). This proposal, which does not have the idea of using a mixture containing a carbonaceous material, cannot attain a sufficient effect.
As described above, the coating of a positive active material has been studied; however, this approach still has difficulty in significantly improving the high-temperature characteristics of nonaqueous electrolyte secondary batteries. Furthermore, a compound oxide of lithium and a transition metal, when used as a positive active material, raises the internal resistance of batteries due to its relatively poor electron conductivity. Unfortunately, the coating may further decrease the electron conductivity of the positive active material depending on the type of the coating material used.